Little is known about the actual component production during WW1. Its quite possible that quantities of certain MkIII-specific parts (ie forends, volley sight plates, cut-offs) had already been produced, or that those sub-contractors had fixed contracts that couldn't easily be cancelled. It could be that LSA was nominated to use up this production, or that they guessed (correctly) that the Army would revert to MkIII specification when the war finished.